1. Field of the Invention
In general, the present invention relates to methods and procedures used to manufacture a matrix of parallel tubes. More particularly, the present invention relates to methods and procedures where multiple parallel tubes are welded to a common base.
2. Prior Art Description
Matrices of parallel tubes are used in many different pieces of equipment. For instance, many heat exchangers use parallel tubes. One of the more exotic uses of a matrix of parallel tubes can be found in hydrogen gas processing cells.
In hydrogen gas processing cells, such as hydrogen purifiers, hydrogen separators, and membrane reactors, tubes of palladium alloy are exposed to gases that contain hydrogen. The palladium alloy is readily permeable to hydrogen at high temperatures. Accordingly, the hydrogen passes through the walls of the tubes and becomes separated from the other gases. An example of hydrogen processing equipment that utilizes a matrix of parallel tubes can be found in co-pending patent application Ser. No. 12/053,528, entitled Hydrogen Gas Separator System Having Micro-Channel Construction For Efficiently Separating Hydrogen Gas From A Mixed Gas Source.
For many reasons, it is desirable to use very small diameter tubes in hydrogen gas processing cells. It is also desirable to place the tubes as close together as possible without touching. In order to hold the tubes firmly in the desired matrix orientation, the various tubes are typically set into a common metal base. In the prior art, holes are drilled into a base. The tubes are then placed into the holes and are brazed into place.
In certain hydrogen gas processing cell applications, hundreds of tubes are set into a single base. The tubes are packed very densely with only a fraction of a millimeter between adjacent tubes. Traditional welding techniques cannot be used because of the minute areas being welded and the lack of available space around each weld. Accordingly, heat brazing techniques are applied.
In a hydrogen gas processing cell, tubes of palladium alloy are brazed to stainless steel end caps. The palladium tubes are then tested for leaks and otherwise conditioned for commercial use. It is only after a palladium tube has been quality tested that it is added to a matrix of tubes. This ensures that the matrix of tubes will not contain any one defective tube that would render the whole matrix defective.
Once tested tubes are added to a matrix, the tested tubes traditionally undergo a second brazing procedure that connects the tubes to the matrix. During the second brazing procedure, the integrity of the pre-tested tubes may be comprised by thermal stresses. Furthermore, during the second brazing process, the initial brazed connections may become undone and create leaks. Additionally, when traditional brazing techniques are used on large matrices of tubes, it has proven difficult to create consistent brazed seals around all of the tubes. Often, some tubes have proper brazed seals, others do not. Since many hundreds of tubes may be present in a single matrix, even a brazing technique with 99.9% effectiveness will result in every hydrogen gas processing cell being defective because of at least one leaking brazed seal.
A need therefore exists for an improved technique for joining hundreds of small diameter tubes in parallel to a common base in a dense matrix. A need also exists for an improved joining process that does not thermally shock the tubes or otherwise compromise the quality of the tubes. These needs are met by the present invention as described and claimed below.